let str:string = 'hello world'
console.log(str)
let passwd:string
passwd = "123456"
console.log(passwd)

let hobby: string[] = ['cook','coding']
let hobby2: Array<string> = ['eat','game']
console.log(hobby,hobby2)

enum Color {Green, Blue, Red}
let c: Color = Color.Blue
console.log(c)

let day: undefined = undefined
let s1: void = undefined

let num1: number = null // 编译正常
let str1: string = null
let num2: number = undefined // 编译正常
let str2: string = undefined

let price: number|null
price = 123
price = null

let type: string| number
type = 123
type = '燃气'